package LanQiaoDemo;

import java.util.Scanner;

public class Demo1 {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        //在此输入您的代码...
        int n = scan.nextInt();//机器人总数

        //不喜欢的数字
        int a = scan.nextInt();
        int b = scan.nextInt();

        //方案总数
        long sum = 0;

        for (int i = 1; i <= n; i++) {
            if (i != a && i != b) {
                sum += method(n, i);
            }
        }
        System.out.println(sum);
        scan.close();
    }

    //计算C几几
    private static long method(int x, int y) {//x在下面，y在上面
        long num1 = 1;
        long num2 = 1;
        int count = y;
        while (count != 0) {
            num1 *= x;
            count--;
            x--;
        }
        count = y;
        while (count != 0) {
            num2 *= y;
            y--;
            count--;
        }
        return num1 / num2;
    }
}
